In an electric-discharge machining apparatus such as a wire electric-discharge machining apparatus or a die sinking electric-discharge machining apparatus employed in precision machining such as mold machining, a high frequency pulsed voltage is applied to a machining electrode and a workpiece while a machining fluid interposes between the machining electrode and the workpiece. In addition, the workpiece is engraved very little by very little by an electric discharge generated by application of this voltage, thereby forming the workpiece into a desired shape.
Types of the machining fluid used in this electric-discharge machining apparatus can be roughly grouped into an insulating oil-containing machining fluid and water-containing machining fluid. Since the water-containing machining fluid is higher in cooling capability than the insulating oil-containing machining fluid, the water-containing machining fluid tends to easily accelerate a machining speed of the apparatus. Whether the water-containing machining fluid or the insulating oil-containing machining fluid, the machining fluid is normally recycled without being disposed of. However, once the machining fluid is used, the machining fluid contains metallic ions produced from the machining electrode and the workpiece or atmospheric components. As a result, a fluid quality differs between the unused machining fluid and the used machining fluid.
It is desirable to stabilize discharge conditions for the electric discharge generated between the machining electrode and the workpiece as much as possible to maintain high machining accuracy of the electric-discharge machining apparatus. To this end, the fluid quality of the machining fluid is controlled so that conductivity (a specific resistance) of the machining fluid falls within a predetermined range in recycling of the machining fluid. If the machining fluid is the water-containing machining fluid, not only the conductivity (specific resistance) but also a hydrogen ion concentration of the machining fluid are controlled according to a material for the workpiece so as not to corrode the workpiece.
For instance, Patent Document 1 discloses an electric-discharge machining fluid circulation supply apparatus configured as follows. If a machining fluid used for electric-discharge machining on a workpiece consisting of special steel or heat-and-corrosion-resistant steel is to be subjected to a regeneration treatment, pH and conductivity of the machining fluid are measured by pH detector and conductivity detector, respectively. Based on these measurement results, the machining fluid is treated by pH regulator (which is specifically an ion exchange treatment device) if it is necessary to do so, thereby regulating the pH of the machining fluid to about six to seven.
Furthermore, Patent Document 2 discloses the following machining fluid-quality control apparatus. In controlling a fluid quality of a used machining fluid, the machining fluid is selectively supplied to a cation exchange unit and an anion exchange unit according to measurement results of pH measuring unit and a specific resistance measuring unit, respectively. A pH and a specific resistance (conductivity) of the machining fluid are thereby controlled within predetermined ranges, respectively.
Patent Document 3 discloses the following method for preventing corrosion of an iron-containing metal although both pH and conductivity of a machining fluid are not controlled by the method. In subjecting an iron-containing metal to wire electric-discharge machining, the machining fluid is circulated and supplied into a column filled with a cation exchange resin, to which one or more type of ions selected from among carbonic acid ions, hydrogen carbonate ions, and hydroxyl ions as well as nitrite ions are fixed. Corrosive ions are thereby removed from the machining fluid without increasing the conductivity of the machining fluid. Furthermore, Patent Document 4 proposes a method for preventing corrosion of a hard metal material or a metal material by adding water-soluble metal anticorrosive consisting of a tetrazol compound or a tetrazol-containing salt to water, thereby preventing corrosion of the hard metal material or the metal material.
Patent Document 1: Japanese Patent Application Laid-open No. S63-191514
Patent Document 2: Japanese Patent Application Laid-open No. H4-141319
Patent Document 3: Japanese Patent Application Laid-open No. 2002-301624
Patent Document 4: Japanese Patent Application Laid-open No. H7-145491